High performance power converters having high efficiency and/or high power density are required in many renewable energy applications, the use of which has significantly increased recently and is expected to continue to increase [1]. Two types of power converters are used in such energy applications, namely inductor/transformer-based converters and switched-capacitor (SC) converters.
Inductor/transformer-based converters contain magnetic components, which are bulky and heavy, so in order to increase the power density and reduce the weight of the converter, the switching frequency can be increased to reduce the size of the magnetic components. However, the power loss of the converter will increase with increasing switching frequency if the converter is designed to work in hard switching operations. Such a power loss should be avoided because it not only decreases the converter's efficiency, but also deteriorates the working environment of the converter due to the additional heat generated. This is especially critical if the power density of the converter is high and there is less area to dissipate the heat. Additional cooling systems may be required for such converters. Alternatively, it is possible to reduce the power loss by employing soft-switching techniques, such as zero-voltage-switching (ZVS), zero-current-switching (ZCS), or both ZVS and ZCS. However, application of these techniques to converters increases the number of required components as well as the complexity of the control. Moreover, application of ZVS and/or ZCS techniques also limits the input voltage and load range of the converter, which is typically undesired for renewable energy applications.
SC converters do not have a magnetic component, and high power density is an inherent feature. Further, with only switches and capacitors in the circuit, an SC converter can easily be fabricated in integrated circuit (IC) form, which can further increase the power density of the SC converter. Therefore, an SC converter can be a good candidate for high power density applications. Currently, the general application of SC converters is mainly for performing voltage ratio transformation or voltage inversion [2]-[5]. They are typically not used for applications requiring voltage regulation. This is because with the existing types of SC converters, voltage regulation is achieved only when the power efficiency is low. To achieve output voltage regulation in SC converters, control methods such as pulse width modulation (PWM) control, pulse frequency modulation (PFM) control, bang-bang control, quasi-switched-capacitor control, and linear control have been applied [6]-[15]. However, no matter which control method is adopted, the operation of SC converters with capacitors at partially-charged state results in an inherent loss of power efficiency [8], [16].
In theory, the efficiency (η) of an SC converter is
      η    =                  V        o                    M        ·                  V          in                      ,where vo is me output voltage, Vin is the input voltage, and M is the voltage gain, which is determined by the topology of the SC converters [8]. A larger deviation of the output voltage Vo from the voltage M·Vin leads to a larger drop of the efficiency of the SC converters. A method to improve the efficiency of an SC converter by combining the SC converter with a configurable voltage conversion process to make the voltage M·Vin, closer to the desired output voltage has been proposed, along with a voltage regulation control process [17]. However, the number of configurations of the SC converter that can be achieved with this approach is small, which limits its application. Reconfigurable SC converters have been proposed, but the number of conversion ratios is limited [19], [41]-[53]. This in turns limit the regulation property of the converter. A unified SC converter has been proposed that can achieve alternating current (AC)—direct current (DC), DC-DC, DC-AC, and AC-AC conversions, and that contains more than 500 conversion ratios [18]. However, this converter requires a large number of switches and capacitors, and this large number of conversion ratios makes the control complex. An SC converter having two variable cascaded SC circuits that achieves variable conversion gain has been proposed [20]. However, due to its cascaded connection, the conversion efficiency is not high. A regulated SC converter with an auxiliary low-drop-out (LDO) converter has been proposed [21]. The SC converter has a fixed conversion ratio, and the SC converter's two input ports are connected to the power source and an LDO output. The regulation of this SC converter's output voltage is conducted via the control of the output voltage of the LDO converter. However, this regulated SC converter is only applicable for step-down conversion, and the conversion ratio must be higher than 0.5. An LDO converter in the related art can be connected in parallel to an SC converter [36]-[39]. The output voltage of the SC converter is controlled by the LDO circuit, and the power is mainly transferred through the SC converter. However, the SC conversion ratio of this SC converter is still low as the LDO controlled output voltage makes the output voltage deviate from the inherent conversion voltage MVin.